Molecular-beam experiments with oriented molecules

Two routes have been followed in reaction stereodynamics. One is to orient a molecular reactant in space and see how the reaction cross-section varies with the molecular orientation. This direction has been pioneered in molecular beam experiments using focusing of an electric hexapole field to control the molecular orientation [221-223a]. Numerous studies have applied this technique to electron-transfer reactions of alkaline-earth atoms [223b]. This technique is now complemented by the so-called brute force technique, where polar molecules are oriented in extremely strong electric fields [83]. 

The beam intensities of oriented molecules using hexapole eleetrie field, however, turn out to be poor because the state selection requires a veiy large flight-length as compared with eonventional molecular beam set-ups. In order to increase the beam intensity, one may propose a way to increase the stagnation pressure of the nozzle. However, the eharaeteristies of the molecular beam such as stream veloeity, rotational temperature and the size distribution of clusters are generally changed [41]. Motivation of the study of Ref.[2] has been to develop a new type of electrostatic state-selector in order to produee an intense oriented molecular beam. Basic idea of this experiment has been that the beam intensity should be simply proportional to the number of beam lines if the moleeular beams can be focused on a point in space. 

Let us consider this case in some detail. If collisions are eliminated in a molecular beam, it is possible to orient molecules (their figure axis) by removing the particles possessing unwanted orientation (analogous to the Stern-Gerlach experiment with a magnetic field). Then, classically, the interaction energy with external electric field is simply... 

The years from 1960 to 1975 represented a golden era in the radiofrequency and microwave spectroscopy of open shell diatomic molecules. Molecular beam electric resonance was one of the most important experimental approaches, but microwave, far-infrared and magnetic resonance studies of bulk gaseous samples were equally important and our understanding of these open shell species is derived from a combination of different experimental approaches. In this book we have chosen to organise our descriptions according to the experimental techniques employed, but as with any such scheme, we run the risk, which we wish to avoid, of not connecting the results from different types of experiment in a coherent manner. As we shall see, the OH radical is the example par excellence which illustrates the pitfalls of an approach which is technique-oriented, rather than molecule-oriented. 

An elegant molecular-beam study of the photofragmentation of aryl halides and methyl iodide has permitted extraction of excited-state lifetimes from a measured anisotropy parameter which depends upon the lifetime of excited state, the rotational correlation time of the molecule, and the orientation of the electronic transition dipole with respect to the —X bond.38 The lifetimes obtained were methyl iodide 0.07 ps, iodobenzene 0.5 ps, a-iodonaphthalene 0.9 ps, and 4-iodobiphenyl 0.6 ps, from which it was concluded that, whereas methyl iodide dissociates directly, the aryl halides predissociate. A crossed-beam experiment using electron-beam excitation has yielded the results for the Si Tt intersystem-crossing relaxation time in benzene, [sHe]benzene, fluorobenzene, and... 

In the interaction of a coherent laser beam with an ensemble of particles (atoms or molecules), one may treat the individual particles as nearly stationary, because even for a fast atomic/molecular beam the particles move only a few micrometres on the time-scale of the photon interaction. Consequently, if the laser photons are absorbed in the interaction, the coherence properties of the laser radiation are transferred to the particle ensemble. It is this coherence transfer that is exploited in experiments such as the orientation of reagents in chemical reactions, or the probing of intramolecular motion in transition states and orientation of products. 

The molecules in an ordinary molecular beam have both rotational and vibrational energy. By suitable modification of the experiment it is possible to produce beams of molecules in a particular vibrational state or with a particular orientation. Therefore one can directly study the effect of vibrational energy or of molecular orientation on chemical reactivity. 

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